This invention relates to a multiplexed system including a plurality of microcomputers associated with watch dog timers for checking run-away in the respective microcomputers.
Multiplexed systems, which have a plurality of microcomputers multiplexed therein, include watch dog timers associated with the respective microcomputer to reset a defective microcomputer so as to stop the program run-away made in the defective microcomputer. For example, Japanese Patent Kokai No. 1-298446 discloses a prior art multiplexed system including two watch dog timers for checking program run-away in first and second microcomputers by monitoring watch dog pulse signals applied thereto from the respective microcomputers. When program run-away occurs in one of the first and second microcomputers, the corresponding watch dog timer produces a reset signal through a reset line to the one microcomputer. The prior art multiplexed system is arranged for the first and second microcomputers to check the watch dog timers at uniform intervals of time. The first microcomputer checks the second watch dog timer by monitoring the reset line of the second microcomputer while interrupting the watch dog pulse signal to the second watch dog timer. The second microcomputer checks the first watch dog timer by monitoring the reset line of the first microcomputer while interrupting the watch dog pulse signal to the first watch dog timer. A1 though the prior art multiplexed system is sufficient for checking the operations of the watch dog timers, it cannot check whether a watch dog timer has initialized the corresponding microcomputer correctly.